The invention relates to the creation of a comfort noise signal. Specifically, the invention relates to a method and system of creating a comfort noise signal in conjunction with an echo canceller.
One consequence of using various types of circuits to couple different types of connections together is the creation of echoes. That is, speech may be reflected by a circuit back to a speaker, causing the speaker to hear his or her own voice.
For example, a near-end device may create a signal (xe2x80x9ca near-end signalxe2x80x9d) that is transmitted to a far-end receiver. The far-end receiver may send a signal (xe2x80x9ca far-end signalxe2x80x9d) back to the near-end device. The far-end signal may include an echo or a spoken reply message, for example. It is desirable to eliminate the effects of echoes. In order to eliminate the effect of echoes, a line echo canceller (LEC) may be used. The echo canceller may create an echo estimate and may subtract the estimate from the near-end signal to create a residual signal.
Besides echoes, another condition that occurs in modern telecommunication systems is the transmission of silence on a line when two speakers are engaged in a conversation. The silence may make one of the speakers believe that the connection between the two speakers no longer exists due to the Non-Linear Processing (NLP) of the LEC. To eliminate this undesirable effect, comfort noise insertion (CNI) may be used to insert background noise onto the line, when silence would normally be present. Inserting the comfort noise in the line has the desirable effect of making each of the speakers believed the line is still connected.
Comfort noise generation may use a nonlinear processing (NLP) scheme to keep a conversation line xe2x80x9calivexe2x80x9d when NLP is activated. As required in Test No. 9 of G. 168 (2000), the generated comfort noise should match the background noise in both frequency content and power level. The changes in the level over time of the inserted comfort noise should match as closely as possible, the level changes that are occurring in the background noise.
ITU-T G.168 highlights the test requirements for comfort noise insertion CNI. CNI is popularly used in CELP vocoders such as those conforming to the ITU-T G.723.1 and G.729 protocols.
The present invention relates to the creation of a comfort noise signal. The present invention advantageously creates a comfort noise signal in conjunction with an echo cancellor. In one aspect of the present invention, a clipping circuit is used to determine when to create a comfort noise signal (and use this signal as an output). In another aspect of the present invention, linear predictive coding (LPC) coefficients are used to model the comfort noise signal and a background power tracking circuit is used to update these coefficients.
In one embodiment of the present invention, a system for forming a comfort noise signal may include a clipping circuit, a comfort noise determination block, and a switch. The clipping circuit may receive a near-end signal, i.e., the near-end signal+echo, a far-end signal and a residual signal. After receiving the signals, the clipping circuit may determine the power of the far-end signal, the near-end signal, and the residual signal. The clipping circuit may determine the ratio of the power of said near-end signal to the residual signal and whether the ratio is less than a predetermined threshold.
The comfort noise determination block may include a LPC analysis module, a synthesis module, a white noise generator, and a scaling module. The comfort noise determination block may be coupled to the residual signal and the far-end signal. The comfort noise determination block may selectively form a comfort noise signal. The switch may be coupled to the comfort noise determination block and the clipping circuit. The switch may be activated to transmit the comfort noise signal whenever the ratio of the power of said the near-end signal to the residual signal is less than a predetermined threshold.
In another embodiment, a second threshold is chosen and if the ratio of powers is not less than the first threshold but less than the second threshold, the residual signal may be attenuated and the attenuated signal used as an output. In yet another embodiment, if neither threshold is met, the residual signal may be used as the output of the system.
These as well as other features and advantages of the present invention will become apparent to those of ordinary skill in the art by reading the following detailed description, with appropriate reference to the accompanying drawings.